Many devices that employ magnetic fields have been encumbered by massive solenoids accompanied by bulky power supplies. However, permanent magnet structures can provide compact, strong, static magnetic fields that do not require the use of additional power supplies. Thus, there has been increasing interest in applications using permanent magnet structures. Often these permanent magnet structures must be designed in unusual configurations. A number of configurations have been designed and developed for electron beam guidance in electron beam tubes of various types. Especially promising for such purposes is the configuration based upon the hollow cylindrical flux source (HCFS) described by K. Halbach in "Proceedings of the Eighth International workshop on Rare Earth Cobalt Permanent Magnets", Univ. of Dayton, Dayton, Ohio, 1985 (pp. 123-136). A hollow cylindrical flux source (HCFS), sometimes called a "magic ring", is a cylindrical permanent magnet shell which produces an internal magnetic field that is more or less constant in magnitude across the central cavity. The field is perpendicular to the central, longitudinal axis of the cylinder, and furthermore the field strength can be greater than the remanence of the magnetic material from which the ring is made. No magnetic flux extends to the exterior of the HCFS structure except at the ends of a finite cylinder. The ideal HCFS is an infinitely long, annular cylindrical shell with a circular cross section. However, the aforementioned Halbach publication discloses an HCFS structure with an octagonal cross section which closely approximates the performance and field pattern of an ideal HCFS. The "HCFS structure" as used herein encompasses not only the ideal cylindrical structure but also other polygonal sided structures which behave with the characteristics of an HCFS.
Recently, HCFS structures have been applied to the design of wigglers and twisters. Reference may be had to "Applications of yokeless flux confinement", J. Appl. Phys. 64(1)), 15 Nov. 1988 for example. More specifically of interest in this regard is applicant's U.S. Pat. No. 4,862,128 entitled "Field Adjustable Transverse Flux Sources" on which the invention of the instant application is an improvement. A wiggler is a radiation source. In wiggler operation, an electron beam is injected into a drift region which is surrounded by a periodic magnet source. The periodic magnet source creates a magnetic field which varies in direction by 180.degree. at fixed intervals, yet is always perpendicular to the principal direction of electron beam travel. A twister is also a radiation source. In twister operation, an electron beam is injected into a drift region in which there is a transverse magnetic field of constant magnitude whose direction changes continuously with progression along the axis, thereby forming a helical field configuration with either constant or progressive pitch. The central cavity of the HCFS structure functions as a drift region in those wigglers and twisters using these HCFS configurations.